I had a very bad shoulder when I was 15. It wasn't exercises that helped me, instead it was changing my racquet. I had been using a racquet that was very light and my coach said that it was creating a slight miss-hit.
Are you using an ultra durable string? I ask because these have also caused me an elbow injury as they pass vibration through your arm.
Do you hit the ball dead centre everytime?
Miss-hitting + Hitting Hard = Higher chance of injury
My racket is 10.7 oz (288 grams) so it could qualify as light. I typically use a polyester string like pro hurricane and a synthetic gut combo, or just polyester string. However I use a rubber band shock absorber so i don't think vibration would be an issue.
Yes, I'm a hard hitter and I mishit a reasonable amount.
Thanks for the info!